iPhone Customize Phone App

I recently watched the show Death Note. If you've seen it, you remember that whenever L contacted anyone, the screen would display an image of the letter L in the font Diploma on a whitish gray background (#B8B8B8 should do). When I accept an incoming call, as long as the number is in my address book, I'd like for the screen to show a similar image, with the first letter of the person's name on the same whitish gray background, and nothing else. Tapping the screen would display the normal in-call menu, and tapping the home button would do the same thing it does in the regular phone application. How doable is this?

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